Gas turbines are known to comprise a compressor, a combustion chamber and a turbine.
Different gas turbines comprise a compressor, a first combustion chamber and a high pressure turbine; thus these gas turbines comprise a second combustion chamber and a low pressure turbine.
In the following particular reference will be made to high pressure turbines, it is anyhow clear that the present invention may be implemented in any kind of turbine, also not being the high pressure turbine or a turbine stage facing the combustion chamber.
Turbines have at least a guide vane row and a rotor blade row.
Each guide vane row is made of stator airfoils having an inner and an outer platform facing respective inner and outer walls of the combustion chamber; moreover the inner and outer platforms are separated from the inner and outer combustion chamber walls by an inner and an outer gap.
During operation the hot gases generated in the combustion chamber from the combustion of a fuel with the compressed air coming from the compressor, pass through the turbine to deliver mechanical power to the rotor.
As known in the art, when hot gases impinge on an obstacle, a high static pressure zone is generated.
Thus, as during operation the hot gases passing through the turbine impinge on the guide vane airfoils, in the zone upstream of the guide vane row a high static pressure zone is generated.
In particular the high static pressure is not uniform, but has peaks in correspondence with the leading edges of the guide vane airfoils.
This effect is particularly relevant in the first guide vane row after the combustion chamber.
In addition, the hot gases path (i.e. the duct wherein the hot gases generated in the combustion chamber pass through) has a first constricting cross section zone followed by a second expanding cross section zone followed by a third constricting cross section zone.
In the second expanding cross section zone a transition between the combustion chamber and the platforms of the guide vane airfoils is provided.
It is clear that this expanding portion makes the hot gases static pressure in the transition zone between the combustion chamber and the guide vane platforms (i.e. in the zone upstream of the leading edges of the guide vane blades) to further increase.
Such high static pressure causes the risk that hot gases enter the gaps, and damage the components nearby (so-called “gas ingestion”).
Because of the particular shape of the hot gases path, this risk is mainly relevant at the inner gap.